It would be like amoebas asking "what do you think the Humans (aliens) in the book looked like?" An amoebas concept of eyes, ears, nose, mouth, skin, symmetry, procreation, emotions, dimensions, etc is widely different than that of a humans. They (and we) have no examples to extrapolate from to even begin to imagine.

We do know that they're corporeal enough, or empathetic enough of other aliens, to be part of a project - spanning galaxies - to prevent the further expansion of the Universe.

That's the physical aspect of the aliens. Psychologically...

As stated earlier, they're communal (grand central station), they interact with other aliens, they're considerate of other aliens feelings (taking on comforting forms), follow Star Treks Prime Directive, etc. Sagans psychology for these aliens was childish. They were gushing over classical music as a testament for the greatness of humanity. That was cringe worthy.

why cringe worthy/childish? If we think about how we'd overcome our own baggage (hint: Religion and woo-woo), our morals should be to minimize misery for ourselves and others. This doesn't mean there won't be any conflicts, but as a morally advanced society, we'd find ways to cope. Aggressive societies will destroy themselves over time, so as stated in the book "leaving them alone" might be the best way to go. And our music (and art in general), apart from mainstream does have its qualities.

"Some part of our being knows this is where we came from. We long to return, and we can, because the cosmos is also within us. We're made of star stuff. We are a way for the cosmos to know itself." (Carl Sagan)